1. Field
The present invention relates to new classes of pixellated and conformal Modulated-Retro-Reflective (MRR) optical devices, including modulated corner-cube devices as well as modulated cat's eye devices. Instead of using a single, large-area modulator, the disclosed modulator utilizes an array of individual pixels, which can be independently controlled or modulated. The array need not be planar and can be formed to any shape as required by the optical design of the specific device structure.
2. Description of Related Art
The novel classes of modulated retro-reflective (MRR) devices disclosed herein may be used for remote sensing, IFF (identification, friend or foe), laser communication links, and optical networks, such as optical relay nodes.
The prior art includes conventional retro-modulation devices using bulk structures, as well as Multiple Quantum Well (MQW) based devices. See U.S. Pat. No. 6,154,299 by Gilbreath et al. of the Naval Research Laboratory entitled “Modulating Retroreflector using Multiple Quantum Well Technology”, the disclosure of which is hereby incorporated herein by reference. The prior art requires fabrication of the entire MQW structure and/or a pixellated array on a large common, planar substrate. For efficient link performance, a large aperture (for example, in the 10 cm range) may be required. Such a relatively large structure demands a very uniform deposition process so that the entire surface has the same band structure and excitonic resonances, etc. In addition, the depth-of-modulation (optical contrast ratio) described in the prior art are dramatically limited by the finite thickness of the thin MQW modulator (typically, 1 μm total thickness). This follows since the MQW component of the prior art is passed either once or twice by the optical beam. By contrast, in accordance with one aspect of the present invention, the optical beam effectively multi-passes the MQW, since it is formed within a Fabry Perot cavity, so the effective number of optical transits can be in the range of 10 to over 100, depending on the design of the cavity Q. Moreover, such a planar structure cannot be flexed or made conformal to curved, generalized surfaces. This would normally limit the ability to retrofit existing large, curved surfaces with a large-area MRR device of such a construction. Moreover, the field-of-view may be limited by such a planar structure, even in the case of a large-area device. In addition, the prior art does not describe how to use a dual-mode device for the MRR structure (for both photodetection and optical modulation). Also, the prior art does not disclose how to fabricate a device with Low Probability of Intercept/Detection (LPI/LPD) in a common element, so that the retro-reflected beam is disabled prior to a successful handshaking procedure (the prior art requires a stand-alone optical shutter to prevent a third party from interrogating the MRR). Finally, the prior art does not disclose a simple method of how to enable the MRR to deal with multiple wavelength indication.
See, also, U.S. Pat. No. 6,455,931 to Hamilton, et al., which is owned by Raytheon Company of Lexington, Mass., the disclosure of which is hereby incorporated herein by reference.
Utilizing a stand-alone shutter to prevent undesirable interrogation of the MRR (as would be the case in the prior art) has a disadvantage since it does not allow the prior art devices to function as a detector and a retro-modulator. Rather, separate detector and modulator assemblies must be utilized. One of the features of the present invention is that while the MRR is in an off-state (that is, while it is effectively shuttered), it can also act as an efficient detector.
The prior art also includes the use of retro-reflectors in communication system. See “Design and analysis of a diffraction-limited cat's-eye retroreflector” by Biermann, Rabinovich, Mahon and Gilbreath, Opt. Eng. 41(7), July 2002, pp 1655-1660, the disclosure of which is hereby incorporated herein by reference.
In accordance with one aspect of the present invention novel classes of MRR devices are utilized for remote sensing, IFF (identification, friend or foe), laser communication links, and optical networks, such as optical relay nodes and a novel fabrication technique. In terms of fabrication and in accordance with one aspect of the present invention, a technique is provided by which such devices can be robustly manufactured so that they can conform to the surfaces upon which are placed (planar, curved, etc. surfaces). The disclosed devices are especially useful because they can be positioned on most curved surfaces (e.g., on the surface of a hemisphere, similar in layout to an eye of a bee or fly). Hence, the overall device can thus accommodate a large field-of-view (FOV) with an overall aperture much greater than that provided by existing fabrication techniques. The disclosed devices can also withstand much greater vibration, acceleration and deformation, since the limiting dimension is now the individual pixel and not the overall wafer dimension or aperture. Yet a further feature of this invention is that a variety of different devices can be fabricated with high yield, owing to the fact that all the pixels can be qualified prior to their self-assembly.
When the MRR devices disclosed herein are in an off state, they are optically opaque to a optical probe, since the disclosed MRR devices then absorb the incoming light while performing a detection function. Also, since the disclosed MRR devices preferably utilize arrays of individual detectors/modulators, the ability to provide a shuttering mechanism which works on an individual detector/modulator basis, certain ones of the detectors/modulators can be shuttered (in an off state) while one or more other detectors/modulators are modulating a probe beam.
The various possible advantages of this invention can be summarized by following (this list is not necessarily all inclusive nor do all embodiments of the invention necessarily enjoy all these advantages):
First, by using a pixellated modulator structure, the prior art compromises between the modulator size for increased data rates and the MRR aperture for increased optical power can be overcome. This allows the design of the individual modulator pixel size for the desired modulation rate and the appropriate number of pixels to cover the required optical aperture.
Second, a Multiple Quantum Well MQW asymmetric Fabry-Perot resonator structure can be conveniently utilized, which results in enhanced on-off contrast ratio and lower voltage operation compared to a conventional transmissive MQW modulator.
Third, the disclosed MQW pixels can operate as both modulators and photodetectors so that only the illuminated pixel(s) need be activated for retro-modulation. This significantly reduces the power dissipation of the MRR. As a result, a “smart” and more secure retro-reflector device can be used to communicate with an interrogator in a selected portion of the FOV of the device, while disabling the device from being interrogated by undesirable third parties appearing in other portions of the FOV of the device. That is, once a handshake procedure has been completed, only the pixel(s) that need be activated (modulated) are activated (modulated), thereby restricting the retro-modulated return for that specified FOV, and disabling a retro-modulated return from being reflected to other parties. This feature also tends to reduce the overall device power consumption of the disclosed device.
Fourth, by using self-assembled pixel transfer technology, the MQW modulator pixels can be positioned on any surface in a predetermined arrangement for optimum MRR optical performance. For example, the cat's-eye retro-reflector architecture, which uses a hemispherical reflecting surface, can be readily realized using the disclosed conformal and pixellated MQW modulator structure. Such a structure can be preformed, such as by casting or molding, and then the individual pixels can be applied thereto using self-assembled pixel transfer technology. This hemispherical MRR structure will greatly enhance its field-of-view independent of its method of manufacture.
Fifth, by utilizing a flexible substrate, the entire retro-device can be conformally attached to a non-planar surface, such as the hemispherical surface mentioned above, thereby enabling the installation of the device onto an existing surface having an arbitrary curvature (e.g., curved platforms and structures) and also enabling effective installation of the device onto a surface which must be curved or conveniently is curved, such as a wing or airframe of an aircraft.
Sixth: Yet another advantage of using the disclosed self-assembled pixel transfer technique for the MRR is that different MQW modulator pixels designed for operation at different wavelengths can be positioned at alternate sites on the retro-reflector surface, hence allowing multi-wavelength MRR operation.
Seventh, by utilizing an Asymmetric Fabry-Perot resonator structure as an integrated detector/modulator, the detection/handshake state of the device can be biased so that, in this mode, the device has optimal detection efficiency. In this mode of operation, nearly all the incident photons can be absorbed in a thin MQW layer. It turns out that in this optimal detection mode, the net specular reflection from the device is also at a minimum. This follows, since the phasing of the beams is such that the Fresnel reflection is canceled out by the Fabry-Perot reflections. In this manner, the specular reflectivity of the MRR is effectively nulled out (all the photons are coupled into the asymmetric Fabry-Perot resonator), resulting in a near-zero retro-reflective “glint” return from the structure (i.e., the retro-reflector is effectively an absorbing structure, with LPI/LPD).
Eighth, the disclosed apparatus can be used for optical communication systems, wireless networks and links, remote sensor nodes and IFF scenarios. Such devices can be employed in myriad free-space applications. It can also be utilized in terrestrial systems as part of rapidly reconfigurable optical links for (i) in-factory transfer of data such as computer-aided design (CAD), (ii) video training, (iii) inventory control, (iv) manufacturing-on-demand information, (v) x-ray data or (vi) any other information requiring high bandwidths.
Ninth, the retro-modulator disclosed herein can also be used in roadside optical information kiosks for automobiles and other vehicles. By placing these potentially inexpensive devices in various locations on highways and city streets (for example, traffic lights), the MRR can relay a variety of traffic and entertainment information using a simple optical probe beam positioned on the car. Alternately, traffic control personnel can use this technology to obtain detailed information about the vehicle and its operating conditions by optical probing. In this case, the retro-modulator will be installed in the car. In yet another vehicular related application, the optical probe is installed in the vehicle for use with optoelectronic-aided auto service to transfer data required to update some electronic components, for example, software or firmware in the vehicle).
Tenth, in terms of a military application, the disclosed MRR is an ideal device for IFF applications where by simply probing a target equipped with the retro-modulator and programmed with the correct code, the target can be easily identified. In addition, these MRRs can be mounted on missiles and/or torpedoes so that an optical beam can easily relay target and tracking information back at the launch site.
Eleventh, the disclosed MRR devices can also be used in airport and airborne traffic control applications for airplane accident avoidance and aircraft identification information. Furthermore, there is a potential for long-range inter-satellite links as well as shorter-range shuttle-to-platform optical links using the disclosed MRR devices. In these cases, the auto-alignment properties of the MRR, coupled with its monolithic, compact architecture, will greatly reduce the prime power requirements, cost, and weight requirements of the system over conventional optical links that employ sources and pointing/tracking subsystems. Finally, in the case of terrestrial applications, adaptive optical techniques can be used to form up the interrogation beam onto the MRR, thereby compensating for wavefront distortions along the path, as well as optimizing the link budget.
It should be noted that there are many aspects of this invention and it will be apparent to those skilled in the art that not all of the features, advantages and applications discussed above will necessarily be applicable to all embodiments.